Field of the Invention
Advances in the design and fabrication of microprocessors in the past few years have resulted in a dramatic increase in processor speeds. In fact average speeds have increased more than four fold in the last decade alone. At the same time peripheral input/output busses have remained substantially constant. For example, the Industry Standard Architecture (ISA) bus operates at 8 MHz while many microprocessors operate in the 100-200 MHz range.
As the discrepancy in speed increases, it has become unfeasible to directly connect the I/O bus to the microprocessor. PC developers have solved this problem by providing "bridge modules" which disconnect the microprocessor/memory from the I/O bus. While this greatly improves the performance of the microprocessor when it is working from memory or cache, it typically does so at the expense of the microprocessor I/O bus interface. In the current PC environment it is typical for a microprocessor to wait 100 or more processor cycles for each I/O bus access.
Under these conditions the amount of time the microprocessor spends handling devices attached to the I/O bus can significantly impact the microprocessor's performance. Many high speed peripheral devices, such as disk drives and network controllers, are designed to be bus masters which allows these devices direct memory access (DMA). This removes the burden of data movement to the peripheral from the microprocessor resulting in an improvement in the utilization of the microprocessor bandwidth. However, microprocessor communication with peripheral devices still remains a major area for improvement.
A critical factor in the performance of today's computer systems is "I/O throughput". This refers to the ability of a computer system to quickly and efficiently move data between main memory and I/O devices.
FIG. 1 illustrates an environment in which the invention operates. High performance I/O devices generally operate as masters on the system bus for moving data from and to main memory. As bus masters, the I/O devices arbitrate for use of the system bus and directly write data to and read data from main memory. Operation of the I/O device is controlled by "device driver" software executing on the computer's main processor. The device driver monitors and controls the I/O devices by reading and writing their I/O registers across the system bus.
Improved I/O throughput can be achieved by making efficient use of the system bus. Improvements in efficiency appear possible in three areas:
1. reduce the number of register I/O write and read operations; PA1 2. reduce I/O device data transfer overhead such as headers and control blocks; and, PA1 3. utilize burst mode transfer capabilities.